This invention relates generally to cable television (CATV) systems and is particularly directed to a two-way CATV system in which upstream data transmission timing is provided by downstream headend generated signals.
CATV systems typically include a central master source of television programming information termed a headend which imparts programs, or groups of programs at different assigned frequencies, to a network of cable connections which, through various distribution and selection network components, ultimately terminate in a branched out plurality of user terminals typically in residences or commercial establishments. CATV program signals are broadcast from the headend to individual subscribers in a "downstream" direction over different assigned carrier frequencies typically from about 50 to 450 MHz. The CATV headend not only transmits television programming information downstream, but may also transmit subscriber address and authorization codes which allow for headend control of subscriber program decode authorization status.
In a two-way CATV system each subscriber is able to transmit signals back to the CATV headend. These subscriber-originated signals may include program purchasing requests, opinion poll responses, and CATV converter status information. These upstream signals make use of out of band frequencies in the 5-32.5 MHz band. The upstream signal distribution network is in the form of a "merging tree topology" in which the signals generated by many sources, or subscribers, converge and are transmitted on a single transmission line back to the CATV headend. Such techniques as signal multiplexing and frequency diversity have been proposed and are utilized to accommodate large numbers of subscribers in a two-way CATV system.
In the upstream signal multiplexing approach the timing of subscriber upstream transmissions is, of course, critical particularly in CATV systems having large numbers of subscribers. Some prior art two-way CATV systems utilize a polling arrangement in which headend generated timing signals typically provided during the vertical blanking interval (VBI) are used to initiate the sequentially timed upstream transmissions of the system subscribers. U.S. Pat. No. 3,769,579 to Harney discloses a CATV system in which each television receiver has a transponder. The transponders are all interrogated by a common start signal, following which a series of clock pulses are provided to the various transponders for controlling their responses. The various periods of reply of the transponders are sequential, with the periods being controlled by the clock signals so that predetermined groups of television receivers reply during predetermined time intervals. The replies from each of the interrogated transponders occur in the period between vertical blanking intervals. U.S. Pat. No. 3,882,392 also to Harney discloses another approach to synchronizing subscriber upstream transmissions in a subscription television system. In this approach the central station (headend) clock is derived from local power frequency and the clock signals at each receiver are derived from the power frequency at the local receiver. Thus, in a hotel or motel situation where the subscription television system operates from a single power source, the central station and all the receivers will operate off of the in-phase local power and the clock signals at both the central station and each receiver will be synchronized. The former approach requires that a transponder be provided for each subscriber, while the latter approach is intended for a television signal distribution system having a common power source which is impractical for most CATV systems which cover large geographic areas and encompass large numbers of subscribers.
A second approach taken in two-way CATV systems involves a contention arrangement in which simultaneous access of the shared media by two or more subscribers is resolved in a predetermined manner. For example, U.S. Pat. No. 4,398,298 to Van Egmond et al. discloses an arrangement in which the transmitting and receiving stages of each station are connected to the cable. The transmitter of a station requesting media access provides a signal of predetermined logic value on the connecting line by changing its output impedance, which logic signal automatically switches off the transmitter of each of the other stations. Another data transmission packet contention system makes use of an approach generally termed "Aloha" which involves the random transmission within a designated time period by those subscribers desiring shared media access. The simultaneous transmission of more than one subscriber results in the collision and mutilation of data packets and unsuccessful headend communications. This is typically followed by another random transmission by the unsuccessful subscriber who may or may not receive a response (acknowledgement) from the headend depending upon upstream data traffic density. In order to increase the rate of successful transmissions, the basic "Aloha" arrangement has been modified into a "slotted Aloha" arrangement wherein the headend transmits a synchronizing signal indicating the beginning of a frame which is divided into a sequence of time slots of mutually equal lengths. Each subscriber transmits a data packet to the headend within a given time slot which is randomly selected from the sequence of time slots. The "slotted Aloha" approach is generally twice as efficient as the basic "Aloha" arrangement in reducing the number of upstream data packet collisions.
One approach to a slotted "Aloha" arrangement is disclosed in U.S. Pat. No. 4,398,289 to Schoute wherein the main station, or headend, determines the number of time slots in the next frame based upon a determination of the number of unmutilated, empty and mutilated time slots in preceding frames. The number of time slots in a given frame is then transmitted as part of the synchronizing signal to each subscriber terminal.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a time-slotted "Aloha" two-way CATV system using contention techniques for upstream data communication in which upstream data transmission synchronization is based on downstream program VBI timing.